FIELD OF THE INVENTION
The present invention relates to an output buffer for semiconductor memory devices, and more specifically to an output buffer circuit for semiconductor memory devices capable of reducing ground noise by isolating ground and pull down transistors, and sequentially driving pull down transistors grounded.
In a conventional output buffer circuit for semiconductor memory devices, pull up and pull down transistors were made in large so as to obtain its large rise-fall output slope.
FIG. 1 is a conventional output buffer circuit for semiconductor memory devices comprised of a plurality of output buffers connected between a power supply voltage and ground in parallel.
In the conventional output buffer circuit for semiconductor memory devices of FIG. 1, x16 output buffers OB1.about.OB16 are connected between the power supply voltage Vcc and ground in parallel to each other. Each output buffer is comprised of a PMOS transistor for pull up MP1.about.MP16 for pull up and a NMOS transistor for pull down MN1.about.MN16, which are serially connected between a power supply voltage Vcc and ground GND. Pull up signals PU1.about.PU16 are respectively inputted into the respective gates of the PMOS transistors MP1.about.MP16 for pull up transistors, and pull down signals PD1.about.PD16 are respectively inputted into the respective gates of the NMOS transistors MN1.about.MN16 for pull down transistors, and the drains of the PMOS transistors MP1.about.MP16 and the NMOS transistors MN1.about.MN16 are commonly connected to generate respective output signals.
The output buffer circuit described above produces output signals by turning on the NMOS transistors MN1.about.MN16 with pull down input signals PD1.about.PD16 while the PMOS transistors MP1.about.MP16 are all turned off by pull up input signals PU1.about.PU16, or, by turning on the PMOS transistors MP1.about.MP16 with pull up signals PU1.about.PU16 while the NMOS transistors are all turned off by pull down input signals PD1.about.PD16.
When the pull down transistors, i.e., NMOS transistors MN1.about.MN16 are turned on by the pull down input signals PD1.about.PD16 simultaneously, a large ground noise is produced.
To solve the above problem, if the semiconductor substrate is a P type, a method for reducing a ground noise that isolates the transistors with the ground by metal lines has been suggested in the prior art. However, this method gives rise to ground noise via pick-up of the substrate, thus causing wrong operation of the output buffer circuit.